Delay circuits or cells are often used to apply a time delay to an input signal, in particular to cause transitions of a digital signal to be delayed. In CMOS (complementary MOS) technology, an inverter is often used as a delay circuit.
Delay circuits or cells having a variable delay are often used in applications where a time constant is to be tuned, such as in DLLs (delay locked loops), ring oscillators, TDCs (time to digital converters), and pulse width modulators. Often, the delay is variable between several fixed time durations, and the complexity of the delay circuit increases as the number of selectable time durations increases. Furthermore, to provide such a variable delay, active components are generally added in the signal path, leading to the addition of jitter.